Thermal performance in square-duct heat exchanger with quadruple V-finned twisted tapes

An investigation on heat transfer augmentation in a square duct with insertion of combined 30 degrees V-fins and quadruple counter-twisted tapes is conducted experimentally. Air as the working fluid enters the test duct having a uniform wall heat-flux condition with Reynolds number ranging from 4000 to 30,000 in the present work. The combined vortex generators (called V-finned counter-twisted tape) are obtained by incorporating V-fins into the edges of quadruple twisted-tapes having a similar twist ratio of 4. The effect of pertinent V-fin parameters such as four relative fin height, (R-B = e/w = 0.16, 0.21, 0.32 and 0.42) and relative fin pitch, (R-P = P/w = 4, 8, 12 and 16) at a single fin attack angle, alpha = 30 degrees on thermal characteristics is examined. The experimental results reveal that the heat transfer and pressure drop in the form of respective Nusselt number and friction factor from the V-finned counter-twisted tape tend to increase with the rise of R-B but show the reversing trend with increasing R-P. The thermal performance of the V-finned counter-twisted tape is considerably higher than that of the quadruple twisted tapes alone. The V-finned counter-twisted tape with R-B = 0.42 and R-P = 4 yields the maximum Nusselt number and friction factor but the one with R-B = 0.21 and R-P = 4 provides the highest thermal performance of about 1.75 at lower Reynolds number. (C) 2015 Elsevier Ltd. All rights reserved.